A conventional Mach-Zehnder interferometer comprises beamsplitters and reflectors for dividing an optical beam into two components that travel along separate but precisely equal optical paths, and for recombining the two beam components to produce an interference pattern. One optical path passes through the "reference arm" of the interferometer, and the other optical path passes through the "object arm" of the interferometer. The interference pattern resulting from recombination of the two beam components provides information about the amplitude and phase distributions of the wavefront of the optical beam.
A Mach-Zehnder interferometer can be made to be "self-referencing" by providing a spatial light filter in the reference arm of the interferometer adjacent the position at which the beam component travelling through the reference arm recombines with the beam component travelling through the object arm. The beam component traveling through the reference arm is called the "reference component," and the beam component travelling through the object arm is called the "information component". The spatial light filter essentially comprises an opaque plate with a pin hole aperture, which functions to remove higher-order aberrations from the reference component of the beam.
In a self-referencing Mach-Zehnder interferometer, the wavefront of the reference component of the beam whose wavefront is being monitored retains only the zeroth-order aberrations that are acquired during passage of the reference component through the atmosphere (or other medium) along the reference arm. One of these zeroth-order aberrations retained by the reference component is the tilt of the wavefront, which is caused by refraction of the reference component by the atmosphere. Tilt of the reference component wavefront is not affected by the spatial light filter.
The wavefront of the information component of the beam is likewise distorted in passing through the atmosphere, but the higher-order aberrations as well as the zeroth-order aberrations are retained by the information component wavefront when the information component is recombined with the reference component. Asymmetries appearing in the interference pattern resulting from recombination of the information component and the reference component provide an indication of the extent of the aberrations imposed by the atmosphere upon the wavefront of the optical beam.
When the information component is recombined with the reference component, the information component is thereby compared (or "referenced") to the reference component. In effect, the aberrated optical beam (i.e., the beam after having passed through the aberration-causing atmosphere) is referenced with respect to a relatively unaberrated version of itself (i.e., unaberrated except for zeroth-order aberrations). The resulting interference pattern provides phase and amplitude information about the aberrated optical beam wavefront, and provides a quantitative indication of the aberrative effect of the atmosphere upon optical beam. However, this phase and amplitude information is distorted due to the zeroth-order aberrations that have not been eliminated from the reference component of the beam.
When a conventional self-referencing Mach-Zehnder interferometer is used to monitor the wavefront of an optical beam, aberrations due to wavefront tilt are generally present in the reference component of the beam in the reference arm of the interferometer. Techniques used in other applications for eliminating aberrations due to wavefront tilt generally involve electromechanical servomechanisms for varying the reflectance angles of mirrors defining optical paths. However, such techniques cannot be used in Mach-Zehnder interferometry. A variation in the reflectance angle of a mirror defining the optical path of a beam component travelling through the reference arm of a Mach-Zehnder interferometer would inherently change the path length of the reference arm, which would violate the requirement of the Mach-Zehnder interferometry that the optical path length of the reference arm remain constant.
Correction of the reference component for wavefront tilt in a self-referencing Mach-Zehnder interferometer was accomplished in the prior art by sequential phase-shifting operations (i.e., "phase-conjugate" techniques) involving complex signal processing that required sampling of the reference component at discrete time intervals. However, until the present invention, there has been no technique for correcting the reference component of a beam whose wavefront is being monitored by a self-referencing Mach-Zehnder interferometer in order to compensate in real time for aberrations due to wavefront tilt.
In applications of self-referencing Mach-Zehnder interferometry for monitoring the wavefronts of optical beams from relatively dim sources (e.g., distant stars, satellite beacons, or military targets), it is necessary to maintain precise alignment of the reference component of the beam with the pin-hole aperture of the spatial light filter in order that the amount of light entering the pin-hole aperture can remain continuously at a maximum. However, unless the alignment of the reference component of the beam with the pin-hole aperture of the spatial light filter can be continuously adjusted in real time, aberrations due to wavefront tilt would vary the amount of light passing through the pin-hole aperture. There has been a need for a technique for providing continuous real-time adjustment of the reference component of an optical beam in a self-referencing Mach-Zehnder interferometer in order to correct for dynamic variations in wavefront tilt.